In most prior systems the flow rate is varied either by acting on the operating speed (i.e. the number of pump cycles per unit time) while keeping the fluid flow rate per pump cycle constant, or by varying the fluid flow rate per cycle while keeping the operating speed constant. More rarely, the flow rate is adjusted by acting in combined manner both on the operating speed and on the flow rate per cycle, as is the case for electromagnetic pumps in which timing circuits act both on the frequency of electromagnet excitation periods and also on the stroke of the electromagnet. Such systems require two data inputs to the pump, i.e. two manual controls or two distinct signal inputs.
The invention is directed more particularly to such known pumps in which a fluid is caused to flow as the result of reciprocating motion of a piston or of cyclical deformation of a mechanically-actuated membrane.
When the pumping member is a piston, the volumetric displacement of the pump is exactly proportional to the stroke of the piston and the resulting flow rate is proportional to said stroke and to the pumping speed. When the pumping member is a mechanically-actuated membrane, a non-linear relationship exists between the volume swept by the membrane and the displacement of the membrane-actuating member which is connected to the center of the membrane. This relationship is complex and depends, in particular, on the manner in which the actuator member is connected to the membrane, on the physical nature of the membrane, on the shape of the membrane seating surfaces, and also on the operating speed. The relationship is generally determined experimentally.
In pumps of the type to which the invention applies, the means for adjusting the volumetric displacement are generally situated in the members for transmitting motor drive to the piston or the membrane actuator, and such means considerably complicate the transmission and make it difficult to transmit forces properly. In addition, if such means are operated automatically it is necessary to supply a non-negligible quantity of power to operate them, and as a result the equipment is expensive and its flexibility in use is reduced relative to devices which can be controlled by small currents using cheap components.
The present invention is the result of researoh into means for making it possible to construct a simple variable-flow metering pump using commonly-available components as much as possible, while still obtaining flow rates which can be adjusted with an error of less than one percent. The present invention provides a method of setting a determined flow rate which enables the metering pump to be driven by a synchronous motor which is itself powered by AC at a fixed frequency, which includes an extremely simple transmission mechanism without any need for a drive-varying device, and which uses an electronic control circuit associated with a microprocessor to provide highly flexible operation at low cost, where all of the above factors are highly advantageous in providing such a variable flow metering pump at low cost. The invention also provides a pump which implements the method.